1. Field of the Invention
The present invention relates to fluid dynamic bearing motors. More specifically, the present invention pertains to fluid dynamic bearing motors such as are used to support and rotationally drive one or more memory discs.
2. Description of the Related Art
The computer industry employs magnetic discs for the purpose of storing information. This information may be stored and later retrieved using a disc drive system. Computer systems employ disc drive systems for transferring and storing large amounts of data between magnetic discs and the host computer. The magnetic discs are typically circular in shape (though other shapes are known), and are comprised of concentric, or sometimes spiraled, memory tracks. Each track contains magnetic data. Transitions in the magnetic data are sensed by a magnetic transducer known as a read/write head. The transducer is part of the disc drive system, and moves radially over the surface of the disc to read and/or write magnetic data.
FIG. 1 presents a perspective view of magnetic media 10 as are commonly employed for information storage. In this view, a plurality of stacked magnetic discs 10′ is shown. The discs 10′ in FIG. 1 are shown in vertical alignment as is common within a disc drive system. Each disc 10 has a central concentric opening 5 for receiving a spindle (shown at 51 in FIG. 2). A rotary motor drives the spindle 51, causing the discs 10 of the disc pack 10′ to rotate in unison.
As noted, the disc 10 itself is supported on a drive spindle 51. The drive spindle 51 rotates the disc 10 relative to the magnetic head assembly 58. FIG. 2 provides a perspective view of a disc drive assembly 50. In this arrangement, a plurality of discs 10′ are stacked vertically within the assembly 50, permitting additional data to be stored, read and written. The drive spindle 51 receives the central openings 5 of the respective discs 10. Separate suspension arms 56 and corresponding magnetic head assemblies 58 reside above each of the discs 10. The assembly 50 includes a cover 30 and an intermediate seal 32 for providing an air-tight system. The seal 32 and cover 30 are shown exploded away from the disc stack 10′ for clarity.
In operation, the discs 10 are rotated at high speeds about an axis (not shown). As the discs 10 rotate, the air bearing slider on the head 58 causes the magnetic head 58 to be suspended relative to the rotating disc 10. The flying height of the magnetic head assembly 58 above the disc 10 is a function of the speed of rotation of the disc 10, the aerodynamic lift properties of the slider along the magnetic head assembly 58 and, in some arrangements, a biasing spring tension in the suspension arm 56.
The servo spindle 52 pivots about pivot axis 40. As the servo spindle 52 pivots, the magnetic head assembly 58 mounted at the tip of its suspension arm 56 swings through arc 42. This pivoting motion allows the magnetic head 58 to change track positions on the disc 10. The ability of the magnetic head 58 to move along the surface of the disc 10 allows it to read data residing in tracks along the magnetic layer of the disc. Each read/write head 58 generates or senses electromagnetic fields or magnetic encodings in the tracks of the magnetic disc as areas of magnetic flux. The presence or absence of flux reversals in the electromagnetic fields represents the data stored on the disc.
In order to accomplish the needed rotation of discs, an electric motor is provided. The electric motor is commonly referred to as a “spindle motor” by virtue of the drive spindle 51, or “hub,” that closely receives the central opening 5 of a disc 10. FIG. 3 illustrates the basic elements of a known spindle motor design, in cross-section. The motor 400 first comprises a hub 410. The hub 410 includes an outer radial shoulder 412 for receiving a disc (not shown in FIG. 3). The hub 410 also includes an inner shaft 414. In this arrangement, the shaft 414 resides and rotates on a stable counterplate 440. A sleeve 420 is provided along the outer diameter of the shaft 414 to provide lateral support to the shaft 414 while it is rotated.
It can be seen that a bearing surface 422, or “journal surface,” is formed between the shaft 410 and the surrounding sleeve 420. In early arrangements, one or more ball bearing systems (not shown) was incorporated into the hub 410 to aid in rotation. Typically, one of the bearings would be located near the top of the shaft, and the other near the bottom. A raceway would be formed in either the shaft or the sleeve for holding the plurality of ball bearings. The bearings, in turn, would be lubricated by grease or oil. However, various shortcomings were realized from the mechanical bearing system, particularly as the dimensions of the spindle motor and the disc tracks became smaller. In this respect, mechanical bearings are not always scaleable to smaller dimensions. More significantly, in some conditions ball bearings generate unwanted vibrations in the motor assembly, causing the read/write head to become misaligned over the tracks. Still further, there is potential for leakage of grease or oil into the atmosphere of the disc drive, or outgassing of the components into this atmosphere.
In response to these problems, hydrodynamic bearing spindle systems have been developed. In these types of systems, lubricating fluid is placed along bearing surfaces defined around the rotating spindle/hub. The fluid may be in the form of gas, such as air. Air is popular because it avoids the potential for outgassing of contaminants into the sealed area of the head disc housing. However, air cannot provide the lubricating qualities of oil or the load capacity. Further, its low viscosity requires smaller bearing gaps and, therefore, higher tolerance standards to achieve similar dynamic performance. As an alternative, fluid in liquid form has been used. Examples include oil and ferro-magnetic fluids. A drawback to the use of liquid is that the liquid lubricant should be sealed within the bearing to avoid leakage. Any loss in fluid volume results in a reduced bearing load capacity and life for the motor. In this respect, the physical surfaces of the spindle and of the housing would come into contact with one another, leading to accelerated wear and eventual failure of the bearing system.
Returning back to FIG. 3, the motor 400 of FIG. 3 represents a hydrodynamic bearing system. A thrust plate 430 is disposed between the shaft 414 and the surrounding sleeve 420. Fluid is injected in gaps maintained between the shaft 414 and surrounding parts, e.g., the counterplate 440, the sleeve 420, and the thrust plate 430. The fluid defines a thin fluid film that cushions relative movement of hub parts.
The motor 400 is actuated by energizing coils in a stator in cooperation with one or more magnets. In the view of FIG. 3, magnets 450 are seen disposed within the hub 410, while stator coils 452 are provided on a base 460. The magnets 450 and stator coils 452 interact to provide rotational movement of the hub 410.
A means for retaining fluid within a hydrodynamically operated bearing surface for a spindle motor is to provide oil pumping grooves in the vertical journal bearing surface between the shaft and the sleeve or in the thrust bearing gap between the shaft and the counterplate. However, in the case of a straight-shaft journal bearing, axial space that could be used for journal bearing surface is rendered ineffective due to its being devoid of oil from the asymmetric pumping action of the seal. Also, since the voided area is not lubricated, bearing damage could result from contact in the non-lubricated area above the grooves during rotational excitement of the spindle. Thus, a need exists for an improved fluid dynamic bearing system for a spindle motor that retains liquid within and along the bearing surfaces. Further, there is a need for such a motor that reduces or eliminates dry contact in the vertical journal bearing surface during rotation of the motor. Still further, there is a need for a hydrodynamic bearing arrangement that reduces the required length of the vertical journal bearing, as would be beneficial in the design of a hard drive for a lap-top computer, where such space is at a premium.